SelectedItem（ListPicker）默认值不在我的属性中（MVVM Light） - Wp7

Question

我有一个列表选择器，我试图通过使用Mvvm方式绑定。它工作但我不明白为什么当List Picker加载一个值时，我的ViewModel中的属性填充为null。

我将该属性设置为null，我可以让它获取我的Collection中的第一个元素，但我想知道为什么没有触发绑定并且以这种方式填充值。

查看

   <Grid x:Name="ContentGrid"
              Grid.Row="1">
            <Grid.Resources>
                <DataTemplate x:Name="PickerItemTemplate">
                    <StackPanel Orientation="Horizontal">
                        <Border Background="LightGreen" Width="34" Height="34">
                            <TextBlock Text="{Binding Country}" FontSize="16" HorizontalAlignment="Center" VerticalAlignment="Center"/>
                        </Border>
                        <TextBlock Text="{Binding Name}" Margin="12 0 0 0"/>
                    </StackPanel>
                </DataTemplate>
                <DataTemplate x:Name="PickerFullModeItemTemplate">
                    <StackPanel Orientation="Horizontal" Margin="16 21 0 20" d:DataContext="{Binding Cities[0]}">
                        <TextBlock Text="{Binding Name}" Margin="16 0 0 0" FontSize="43" FontFamily="{StaticResource PhoneFontFamilyLight}"/>
                        <TextBlock Text="language: "/>
                        <TextBlock Text="{Binding Language}" Foreground="Green"/>
                    </StackPanel>
                </DataTemplate>
            </Grid.Resources>
            <toolkit:ListPicker  x:Name="listPicker" ExpansionMode="FullScreenOnly"  ItemTemplate="{StaticResource PickerItemTemplate}"    FullModeItemTemplate="{StaticResource PickerFullModeItemTemplate}"  Header="Cities" FullModeHeader="Cities"  CacheMode="BitmapCache" Height="110" Margin="12,12,12,0" VerticalAlignment="Top" ItemsSource="{Binding Cities}" SelectedItem="{Binding SelectedCity, Mode=TwoWay}"/>
        </Grid>
    </Grid>

查看模型

 public class MainViewModel : ViewModelBase
    {
        public ObservableCollection<City> Cities { get; set; }


        public MainViewModel()
        {
            if (ViewModelBase.IsInDesignModeStatic)
            {
                Cities = new ObservableCollection<City>
                {
                    new City
                    {
                         Name = "Vancouver",
                        Language = "En",
                         Country = "Canada"
                    },
                    new City
                    {
                        Name = "Las Vegas",
                        Language = "En",
                        Country = "US"
                    }
                };
            }
            else
            {
                // live data would go here but in this example, It will be more hardcoded data.

                Cities = new ObservableCollection<City>
                {
                    new City
                    {
                         Name = "Toronto",
                         Language = "En",
                         Country = "Canada"
                    },
                    new City
                    {
                        Name = "Reno",
                        Language = "En",
                        Country = "US"
                    }
                };

            }
        }

        /// <summary>
        /// The <see cref="SelectedCity" /> property's name.
        /// </summary>
        public const string SelectedCityPropertyName = "SelectedCity";

        private City selectedCity = null;

        /// <summary>
        /// Sets and gets the SelectedStore property.
        /// Changes to that property's value raise the PropertyChanged event. 
        /// </summary>
        public City SelectedCity
        {
            get
            {
                return selectedCity;
            }

            set
            {
                if (selectedCity == value)
                {
                    return;
                }

                RaisePropertyChanging(SelectedCityPropertyName);
                selectedCity = value;
                RaisePropertyChanged(SelectedCityPropertyName);
            }
        }

    }